Known in the art is a process for simultaneously producing a solution of sodium hydroxide and chlorine by way of an electrochemical decomposition of a solution of sodium chloride in an electrolyzer with a filtering diaphragm, while maintaining sodium chloride concentration within the range of from 240 to 310 g/l (4.1 to 5.3 mol/l) and pH values within the range of from 0.2 to 4.5. The process is conducted at a temperature of from 95.degree. to 104.degree. C.
The electrolysis is effected by continuously passing a solution of sodium chloride through the diaphragm in the direction of from the anode to the cathode at a rate of from 10.0 to 13.5 ml/hr per 1 A of the electrolyzer load. The degree of electrochemical decomposition of sodium chloride (the degree of conversion of chloride to sodium hydroxide) is equal to 0.50-0.53.
To maintain a high concentration of sodium chloride in the electrolyzer, use is made of a feeding solution with a concentration of 315-330 g/l (5.4-5.65 mol/l) of NaCl. The feeding solution supply into the electrolyzer is effected with an excess relative to the solution passage through the diaphragm.
The excessive amount of the solution of sodium chloride is passed to the stage of after-saturation with sodium chloride, then recycled back to the electrolyzer.
According to experimental data, the current yield of sodium hydroxide (alkali) is 93.9-97.2% at the following ranges of the process parameters: concentration of the solution of sodium hydroxide of 140-260 g/l (3.5-6.5 mol/l), concentration of sodium chloride in the electrolyzer of from 240 to 300 g/l (4.1 to 5.3 mol/l), value of pH of the anolyte of from 2.1 to 4.05 (cf. U.S. Pat. No. 3,403,083 Cl. 204-98, published Sept. 24, 1968).
A disadvantage of this prior art process resides in the production of a relatively diluted solution of sodium hydroxide with a high content of the residual sodium chloride-up to 200 g/l, high rates of consumption of heating steam for evaporation of the resulting electrolytical liquors for the preparation of a concentrated solution of sodium hydroxide with a content of NaOH of from 600 to 650 g/l (15.0-16.5 mol/l).
Also known in the art is a process for a simultaneous production of a concentrated solution of an alkali metal hydroxide, for example sodium hydroxide, and chlorine by way of an electrochemical decomposition of an alkali metal chloride in an electrolyzer with a filtering diaphragm upon passing the solution of sodium chloride through the filtering diaphragm.
Fed into an electrolyzer is a concentrated solution of sodium chloride containing 290 to 315 g/l (5.0 to 5.4 mol/l) of NaCl. The process is conducted at the temperature of 90.degree. C., pH value below 1.5 (within the range of from 1.0 to 1.2) and the degree of electrochemical decomposition of the alkali metal chloride (the degree of conversion of the chloride to hydroxide) of from 0.55 to 1.0. Calculations show that the minimum rate of the solution of sodium chloride supply into the electrolyzer (corresponding to the rate of passage of the anolyte through the diaphragm under the electrolysis conditions without recycling of the anolyte) in the prior art process is 6.5 ml/hr per 1 A of the electrolysis current. The degree of electrochemical decomposition of sodium chloride is 0.55 to 1.0 is equivalent to the production of a solution of an alkali metal hydroxide with a concentration of not less than 200 g/l. The maintenance of the alkali metal hydroxide concentration at a level of 350 to 780 g/l in the prior art process (without recycling and after-saturation of the anolyte) can be effected only by replenishing the catholyte by means of a concentrated solution of sodium hydroxide or a solid alkali, or a partial evaporation of the catholyte, followed by its recycling to the cathodic space of the electrolyzer.
The content of the alkali metal chloride in the anolyte during electrolysis ranges from 2.5 to 4.0 mol/l (150-250 g/l) of sodium chloride.
An average current yield is 93 to 95%.
The prior art process has a disadvantage residing in a high content of the impurity of an alkali metal chlorate in the resulting solution of the alkali metal hydroxide, the necessity of recycling and after-saturation of the latter solution to maintain a high concentration thereof.
The prior art process most resembling that of the present invention in its subject matter and the result obtained is a process for producing a concentrated solution of an alkali metal hydroxide, for example, sodium hydroxide, and chlorine by way of an electrochemical decomposition of an alkali metal chloride, e.g. sodium chloride in an electrolyzer with a filtering diaphragm, while passing a solution through the diaphragm and a degree of decomposition of the chloride of from 0.55 to 1.0 and maintaining the alkali metal chloride concentration in the electrolyzer of from 4.3 to 5.3 mol/l and the anolyte pH value of from 1.5 to 2.5.
The electrolysis is conducted at the temperature of 90.degree. C. During the electrolysis a solution of an alkali metal chloride, e.g. sodium chloride, is recycled through a vessel, where in it is after-saturated with the alkali metal chloride and acidified. To obtain a high-concentration solution of the alkali metal hydroxide directly in the electrolyzer, into the cathodic space during the electrolysis there is introduced a solution of sodium hydroxide with a concentration of from 700 to 750 g/l. The calculations show that the flow rate of the anolyte through the diaphragm is at least 7 ml/hr per 1 A of the electrolysis current.
The average current yield of sodium hydroxide is 96.3%. The content of impurities of sodium chlorate in the obtained sodium hydroxide is 0.22-0.42 g/l, that of sodium chloride--not less than 6-10 g/l. (cf. USSR Inventor's Certificate No. 831869 "Process for producing concentrated solution of alkali metal hydroxide and chlorine", by V. L. Kubasov, L. I. Yurkov, A. F. Mazarko, F. I. Lvovich and M. A. Melnikov-Eichenwald Published May 23, 1981).
This prior art process has a disadvantage residing in a high content of the impurity of sodium chlorate in the resulting solution of an alkali metal hydroxide and complicated character of the process associated with the necessity of admission of sodium hydroxide into the cathodic space of the electrolyzer.